Lean-burn gasoline and diesel engines are operated at higher than stoichiometric air-to-fuel (A/F) mass ratios for improved fuel economy. They also provide good driving performance and reduced carbon dioxide emission compared with stoichiometric gasoline engines. Such lean-bum engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed up diesel engine is typically in the range of 200° to 400° C. and has a representative composition, by volume, of about 10-17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx, and the balance nitrogen and water. These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content and the water content in the hot exhaust stream.
Traditional three-way catalysts used with stoichiometric gasoline engines are not very effective for treating lean-burn exhaust. Researchers have attempted to find durable catalysts that can selectively reduce NOx using engine-out hydrocarbons (HC-SCR) despite the competing combustion reaction with the oxygen content of the exhaust. Such catalyst developments have not been successful because of poor activity, narrow operating temperature window and insufficient durability of candidate catalyst materials.
One of the technologies being considered for the lean-burn gasoline or diesel engine NOx emission control is to reduce NOx using selected fuel-component hydrocarbons added to the exhaust stream. The hydrocarbons are intended to provide chemical species for reduction of nitrogen oxides to nitrogen. In such hydrocarbon-assisted SCR, ethyl alcohol is viewed as having reductant-utility like a hydrocarbon because it can be converted in the exhaust to chemical species useful in the reduction of NOx. Ethyl alcohol can be delivered as a fuel additive, and, if desired, easily distilled off from the fuel and stored in a separate tank on a vehicle.
In a refinement of this HC-SCR approach developed by the assignee of this invention, ambient air is passed through a non-thermal plasma generator to produce ozone which is also introduced into the exhaust stream for oxidation of NO to NO2 which has been easier to convert to N2. Ozone, together with oxygen in the exhaust promotes limited oxidation of the hydrocarbons to aldehydes and alcohols, preferably to compounds containing more than two carbon atoms. In plasma-assisted HC-SCR systems, partially oxidized HCs like aldehydes and alcohols (>C2) have been shown to be very effective in reducing NOx over a dual bed of BaY zeolite (upstream bed) and CuY zeolite (down stream) catalysts. However, a high NOx conversion efficiency can be obtained only with a large volume of such catalysts, which results in packaging problems and slow warm-up.
A catalyst consisting of finely divided particles of silver dispersed on high surface area gamma-alumina particles (Ag/Al2O3) has been shown to be moderately effective in reducing NOx with various, hydrocarbon species such as engine-out HCs, partially oxidized HCs and fuel-component HCs. The use of a non-noble metal catalyst is preferred for NOx conversion for a lower cost process. Further, Ag/Al2O3 can reduce NOx at higher space velocities than the base metal cation-exchanged zeolite catalysts. However, the use of the silver catalyst alone failed to convert all NO to N2 and produced unwanted by-products in the exhaust.
It is, thus, an object of the present invention to provide an improved method of practicing hydrocarbon-assisted selective catalytic reduction in such oxygen-rich and nitrogen oxide-containing exhaust mixtures. It is a more specific object of the present invention to provide an improved method of utilizing silver catalysts in the practice of HC-SCR.